Nuclear magnetic resonance (NMR) spectroscopy (NMR spectroscopy) is one of the most important spectrographic methods for elucidating the structure and dynamics of molecules, in particular in organic chemistry and biochemistry. However, the sensitivity of NMR spectrometers reaches its limits in many applications, for example, in investigation of large biomolecules in vitro and in vivo. The lack of sensitivity can be improved to a certain extent by applying a higher external magnetic field, but this is possible only to a limited extent and involves very high efforts.
A very promising alternative for increasing the sensitivity of NMR measurements in biomolecules, for example, consists of a method known as “dynamic nuclear polarization” or the “DNP method” according to the abbreviation of the English term “dynamic nuclear polarization.” DNP results from the transfer of the spin polarization of the electrons to the nuclei according to the principle also known as the “Overhauser effect.” To make DNP usable in NMR spectroscopy, the electronic spin polarizations must first be transferred to the nuclear spin system. To do so, the sample is excited at an electronic spin resonance frequency, usually referred to as the EPR frequency, where EPR is the abbreviation for the English term “electronic paramagnetic resonance.” The EPR frequency, also known as the Larmor frequency, corresponds to the splitting of the energy of electronic spin energy quantum states of an atom or molecule in an external magnetic field according to the Zeeman effect, which would degenerate without an external magnetic field. The splitting of the energy states is proportional to the strength B of the external magnetic field and thus the value of the EPR frequency is a function of the magnetic field strength. However, in applications that are of practical relevance, this is always in the microwave range. The change of the polarization of the electronic spin through input of EPR microwaves is often referred to graphically as “pumping.”
The NMR signal gain due to DNP is proportional to the square of the intensity of the EPR microwave field as long as the EPR transitions are not saturated. To obtain an EPR microwave field with the highest possible power and/or field strength, microwave resonators in which the sample is arranged for stimulation of the EPR transitions are preferably used.
As in EPR, nuclear magnetic resonance (NMR) is also based on transitions between quantum states of a spin in an external magnetic field, with the difference being that energy splitting of the nuclear spin is much smaller than in EPR. The NMR frequencies are typically in a two-digit megahertz range, i.e., still in the high-frequency (HF) range. Instead of the term “high frequency,” the literature also uses the term “radio frequency.” The term “high frequency” should therefore not hide the fact that these NMR frequencies are of course the lower frequencies of the frequencies involved, namely lower than the aforementioned microwave frequencies.
Since a high-intensity HF field is also necessary for NMR spectroscopy, an HF resonator in the form of an HF resonance coil is generally also used. Therefore so-called double-resonance structures, which have a microwave (MW) resonator for EPR transitions and an HF coil for NMR transitions are available for DNP-NMR experiments, so that the same sample may be exposed simultaneously to an MW field and an HF field, each with a high intensity.
One method that is conceptually related to DNP-NMR spectroscopy is the so-called electron nuclear double-resonance spectroscopy, also known as ENDOR spectroscopy. ENDOR spectroscopy is a special type of EPR spectroscopy, in which NMR transitions in the sample are created by input of HF fields. To this extent, ENDOR spectroscopy is conceptually very similar to DNP-NMR spectroscopy, except that in this case, it is pumped using HF fields, and EPR spectroscopy is performed. A double-resonance structure is also used for ENDOR experiments.